Smoker isn't totally impossible. His story has all been about him trying to work out what true justice is. He's seen (begrudgingly) from Luffy that not all pirates are bad, he's seen from the Warlords that the World Gov aren't infallible. His big friend in the Marines, Kuzan, is questioning his own justice and went a totally different route. Feasibly, you could see a situation arising where, when confronted with evil within the Marines themselves (which hasn't happened to him yet. Vergo was notably considered by Smoker a pirate, not a real Marine) Smoker decides his justice can only be found through Luffy changing the world.
Having said all that, I'd only give a... 2-5% chance that actually happens. Smoker is going to end up on Luffy's side, but not as an official ally.